Electric power is usually supplied in the form of a voltage source, where the supply voltage is maintained constant while the supply current varies with the loading condition. Two or more voltage sources (v1, v2 . . . vn) can be connected in series, as illustrated in FIG. 1a, to provide a higher supply voltage. Parallel connection of voltage sources, on the other hand, is difficult because a slight mismatch of the voltages can result in large current unbalance among the parallel sources.
Sometimes, current-source behavior is required or preferred by certain loads for which the supply current has to be maintained constant while the supply voltage can fluctuate according to the loading condition. Two or more current sources (i1, i2 . . . in) can be connected in parallel, as illustrated in FIG. 1b, to provide a higher supply current. However, series connection of current sources is difficult because of the need to precisely match the currents.
Renewable energy (such as wind and solar) and other distributed generation (DG) sources are usually integrated into the power grid through a power electronics interface that provides power conditioning and control of the raw source output. The common practice is to control the interface to behave like a current source that injects a certain amount of current into the power grid depending on the amount of power the source is able or required to produce. Current-source behavior is desirable in this case because it avoids conflicts with the grid which is a voltage source.
In practice, the grid is not an ideal voltage source due to the series impedances of the lines, transformers, and generators. Likewise, the interface cannot be made a perfect current source which, by definition, has infinite output impedance. In the presence of such non-ideal effects, the stability of the DG-grid system requires that the ratio of the grid impedance to the interface output impedance satisfy the Nyquist criterion. See J. Sun, “Impedance-based stability criterion for grid-connected inverters,” IEEE Transactions on Power Electronics, vol. 27, no. 11, pp. 3075-3078, November 2011.
Multiple solar or wind sources can be connected to the power grid through their own grid interfaces, effectively operating in parallel with each other as current sources. There is an increasing need to operate such sources in series as a way to reduce the cost and increase the efficiency of the power conditioning interface. There are several examples for such applications:                1) Multi-Terminal DC Transmission for Offshore Wind: DC transmission is favored over ac transmission for offshore wind because of the difficulties associated with transferring power over long undersea ac cables. For this application, as illustrated in FIG. 2a, the series-DC architecture (10), in which each wind turbine (12) is controlled to produce a DC output and multiple such DC terminals (14) are connected in series to build a sufficiently high DC voltage for transmission to the onshore power grid, is advantageous as it eliminates redundant DC-AC (16) and AC-DC (18) conversion stages required when an AC offshore collection bus (24) is used, as illustrated in FIG. 2b.         2) Direct Connection of Solar Inverters to Medium-Voltage Distribution Lines: Commercial (large-scale) solar installations are usually integrated to the power grid by connecting to a medium-voltage (e.g. 15 kV or 33 kV) distribution line. To avoid the need for high-voltage electronics, as illustrated in FIG. 3a, the output of each solar inverter (26) is kept at a low voltage level (e.g. 480 V), and a medium-voltage transformer (28) is used to step up the voltage for connecting to the distribution grid or network (30). An alternative architecture, depicted in FIG. 3b, is to connect multiple solar inverters (26) in series such that they can interface directly with a medium-voltage distribution network (30) without a bulky step-up transformer.        3) Modular Low-Voltage Microinverters: As illustrated in FIG. 4a, a microinverter (32) generates 50/60 Hz AC from the DC output of a single solar panel (34). Since the panel output voltage is usually a fraction of the peak voltage of the power grid (36), e.g. 160 V in residential buildings, a microinverter requires a transformer to step up the voltage, as well as the use of high-voltage devices that can withstand the peak grid voltage. The use of a transformer and high-voltage devices can be avoided by designing each microinverter to generate a voltage lower than the panel output voltage and by operating multiple microinverters (32) in series for connecting to the unity grid (36), as depicted in FIG. 4b. This scheme is similar to that shown in FIG. 3b, and reduces the cost and increases the efficiency of the microinverters.        
Common to these three examples are that a) multiple power sources (referred to as sending terminals hereinafter) are connected in series to form a string, and b) the string is connected in parallel with a DC or AC power grid which can be considered a voltage source. To facilitate the series connection, individual sending terminals should act as voltage sources, while the string should behave like a current source in order to operate in parallel with the grid, which is a voltage source.
Such mixed voltage and current source behavior between individual sending terminals and the overall string could be achieved by a central controller when all sending terminals are located in close proximity, e.g. in the same housing. In that case, as depicted in FIG. 5, the central controller (40) would control the total string voltage by regulating the output voltage of individual sending terminals (42, 42′), and an inductor (L, L′) could be inserted in series to absorb the instantaneous voltage difference between the string and the grid, effectively turning the string into a controlled current source.
This central control scheme can work in both DC and AC networks, but requires a high-speed communication link between the controller and each sending terminal, which is difficult to implement when the sending terminals are dispersed, such as in the three examples discussed above, where the physical dimension of a string may range from several meters for rooftop microinverters to several kilometers for offshore wind turbines. The speed of the central control must be comparable with the switching frequency of the semiconductor devices used in each wind turbine or solar converter, such that a loss of control signal for even a few milliseconds may result in destructive damage and bring down the entire string. The central control is also difficult to modularize and to maintain. Therefore, a different approach is needed in order to enable such series connection of renewable sources into the power grid.